Abstract. The Model of Acidification of Groundwater In Catchments (MAGIC) has been calibrated to 94 catchments in Wales, enabling the quantification of regional response to reduced emissions under the second sulphur protocol and incorporating the potential impact of future changes in afforestation. To calibrate the model the methodology utilises, for each site, the best available data derived from databases which are consistent across the whole region but which vary in spatial resolution. The calibrated model closely matches observed stream water chemistry across the region. The predicted future response is a reduction in acidity of surface waters in the region. The recovery is less marked at forested catchments, and some forested sites undergo a further decrease in acid neutralising capacity (ANC). The predictions from the multiple-site method are statistically consistent with those produced by an alternative regional modelling method which is based upon Monte-Carlo procedures. The multiple-site method, however, has the advantage of providing future predictions which are also spatially distributed across the region.